1. Field of the Invention
The present invention relates to systems and methods for dynamically compressing images to be transmitted in drilling situations.
2. Description of the Related Art
Boreholes are drilled deep into the earth for many applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. As the borehole is drilled, images of the formation surrounding the borehole can be formed. These images are often referred to as “resistivity images.”
Resistivity images can be formed by utilizing transmitter/electrode sensor configuration that measures formation resistivity near the borehole. The resistivity images can be used to identify, for example, faults and formation bed boundaries and may be formed with either a wireline tool (wireline measurement) or while drilling (measuring while drilling; MWD). In the case of wireline measurement, the image can be transferred at or near full resolution in real-time due to the large bandwidth of the wireline. In MWD situations, the communication bandwidth may be limited by the communication transmission channel. For example, in transmission channels with low bandwidth, such as the mud utilized in mud pulse telemetry, in order to transmit the data in a timely manner, the raw images formed by an MWD tool need to be compressed before being transmitted to the surface. The received, compressed images can then be decompressed at the surface to “recreate” the raw images.
There are a number of compression methods, all with their advantages and disadvantages. The joint photographic experts group (JPEG) method is one method to compress an image. The JPEG method is a lossy compression method that loses some of the data related to the image during the compression. In very low bandwidth telemetry systems, the JPEG method has proven to provide the images in real-time with acceptable quality. However, some problems can occur. For example, and as explained more fully below, the nature of the JPEG methods causes more detailed (less homogenous) images to degrade more than less detailed images.
According to the JPEG method, the transmission of an image includes transmitting a header that identifies the image that is to follow. For instance, in some cases, the header can include a time stamp or other identifying indication. The compressed image data follows the header. One property of the JPEG method is that the main features of an image are always transmitted first and the “details” are sent later in a bit-by-bit manner. The more data that is received, the more detail the decompressed image has. In operation, a MWD tool acquires an image, compresses the image and then transmits the information. In many applications, a certain time slot or percentage of the bandwidth of the transmission channel is assigned to transmitting the images. Thus, the transmission rate of the transmission channel limits the amount of detail that can be transmitted per image. That is, in MWD applications where the transmission rate is substantially constant, every compressed image will be transmitted with approximately the same number of data words regardless of the amount of detail. As a consequence, images with a lot of details can experience greater compression losses than more homogeneous ones.